The present invention relates to the synthesis of compounds having the formula: EQU R.sub.y MX.sub.(3-y) ( 1)
wherein R is lower alkyl, M is arsenic or phosphorus, X is a halogen, and y is 1 or 2. (R, M, and X retain these definitions throughout this specification, unless the context indicates otherwise.) The invention also relates to methods for preparing compounds of the formula: EQU R.sub.y MH.sub.(3-y) ( 2)
wherein H is hydride and the other substituents are like those of formula (1), from compounds of formula (1).
Compounds of formula (2), have recently found favor as reactants for metal organic chemical vapor deposition of III/V compound semiconductor films for electronic, optical, and other technologies. The utility of such arsenic and phosphorus compounds for metal organic chemical vapor deposition is disclosed in U.S. Pat. No. 4,734,514, issued to Melas et al. on Mar. 29, 1988. That patent is hereby incorporated by reference herein in its entirety.
While the alkyl hydrides of formula (2) are very useful compounds, their synthesis has been long and complicated, with a low yield. In Example 1 of the Melas patent previously incorporated by reference, the synthesis of diethylarsine using arsenic trichloride as a starting material requires a sequence of four reactions. The first three reactions are required to form diethylchloroarsine -- a compound according to formula (1). Thus, one problem facing the art has been how to form the compounds of formula (1) more directly and with higher yields.
The arsenic or phosphorus compounds of formula (1) have been reported to be synthesized by directly reacting the corresponding alkyl halide with arsenic or phosphorus at 70.degree. C. in the presence of copper as a catalyst, according to the equation; ##STR1## (L. Maier, Inorganic Synthesis 7:82 (1963).) (The antimony synthesis hasn't been reported.) Unfortunately, the yield of the halide has been reported to be quite low, particularly when is arsenic and the R group is ethyl.
Another reaction scheme which provides formula (1) halides is found in Kharasch, et al., J. Org. Chem. 14, 429 (1949): EQU R.sub.4 Pb+2 AsCl.sub.3 .fwdarw.R.sub.2 PbCl.sub.2 +2 RAsCl.sub.2( 4) EQU R.sub.2 PbCl.sub.2 +AsCl.sub.3 .fwdarw.RAsCl.sub.2 +PbCl.sub.2 +RCl(5)
This can be a one-step synthesis. The analogous synthesis for phosphorus is: EQU Et.sub.4 Pb+3PCl.sub.3 .fwdarw.3 EtPCl.sub.2 +PbCl.sub.2 +EtCl(6)
However tetraalkyl lead compounds are toxic, and lead as an impurity might damage III/V films formed from the resulting product.
Another multistep synthesis of formula (1) halides is found in Burton, J. Chem. Soc. 450 (1926) and Gibson, et al., J. Chem. Soc. 2518 (1931), as follows: ##STR2## (In the formulas herein, "Ph" is phenyl).
The yield of this reaction sequence is low.
Other, less pertinent syntheses of the formula (1) halides are also known. (See Doak & Freeman, Oranometallic Compounds of Arsenic, Antimony, and Bismuth, John Wiley & Sons, Inc., 1970).
The following two reactions are known for triphenylarsine (G. D. Parkes, R. J. Clarke and B. H. Thewlis. J. Chem. Soc. 429 (1947); A. G. Evans and E. Warhurst, Trans Faraday Soc., 44. 189 (198); H. D. N. Fitzpatrick, S. R. C. Hughes, and E. A. Moelwyn-Hughes, J. Chem. Soc. 3542 (1950)), and both monochloro and dichloroarsine derivatives can be made this way: EQU 2 (Ph).sub.3 As+AsCl.sub.3 .fwdarw.3 (Ph).sub.2 AsCl (9) EQU (Ph).sub.3 As+2 AsCl.sub.3 .fwdarw.3 (Ph)AsCl.sub.2 ( 10)
However, these reactions are unknown for alkyl arsines, and chloroalkylarsines cannot be made in this manner. For example, the reaction of trimethylarsine with arsenic trichloride has been reported to form only the stable addition compound: EQU (CH.sub.3).sub.3 As.AsCl.sub.3 ( 11)
(A. Valeur and P. Gaillot. Bull. Soc. Chim. Fr., 41. 1318 (1927).
While not intending to be bound by this theory, the inventors believe that trialkylarsines have not previously been recognized as being reactive in the present context because triphenylarsine is far less basic than trialkylarsines. As a result, triphenylarsine does not form a stable adduct with arsenic trichloride and is readily reactive with additional arsenic trichloride or triphenylarsine to form products according to formula (1) above. On the other hand, attempts to form the products of formula (1) directly from the corresponding alkylarsine and trihaloarsine have failed because a stable adduct of these reactants forms and does not easily react to form the desired products.
Redistribution reactions take place readily for both alkyl and aryl derivatives of bismuth. This perhaps may be attributed to the weak bismuth to carbon bonds in these compounds facilitating the exchange of R and X (halogen) groups. (A. Marguardte, Berichte 20 1516 (1887)). EQU Me.sub.3 Bi+2 BiBr.sub.3 .fwdarw.3 MeBiBr.sub.2 ( 12) EQU Et.sub.3 Bi+2 BiBr.sub.3 .fwdarw.3 EtBiBr.sub.2 ( 13) EQU 2 Ph.sub.3 Bi+BiCl.sub.3 .fwdarw.3 Ph.sub.2 BiCl (14)
Both primary and secondary arsines of formula (2) are generally prepared by reducing a different arsenic compound with a reducing agent. Thus, alkylarsonic (or alkylarsinic) acids and alkylchloroarsines are common arsenic starting sources which can be reduced with zinc dust, zinc amalgam or zinc-copper couple in aqueous hydrochloric acid. For example; ##STR3## See W. R. Cullen and W. R. Leeder, Can. J. Chem., 47 2137 (1969))
Lithium aluminum hydride has also been used, but the results are generally less satisfactory with poorer yields: ##STR4## (See E. Wiberg and K. Modritzer, Z. Naturforsch, B, 11. 751 (1956) and B, 12, 127 (1957))
Arsines made by the routes of equations (15) and (16) might be contaminated with zinc, mercury or copper and subsequently might damage the III-V films formed from these products. The water used in reaction (16) can produce oxygen-containing impurities in the films.